Shielding is an effective method of protecting communications cables from electrical and magnetic disturbances which arise from external sources and which result in noise, for example. The use of shielding, particularly for small pair size cables in which a relatively high signal to noise ratio is desirable, has increased greatly in recent years. These cables generally include a core comprising one or more signal carrying conductors and provisions for shielding the conductors from parasitic electrical and magnetic fields. Not only should the shielding prevent external disturbances from adversely affecting the signal being carried by one or more of the core conductors, but also it is intended to prevent the leakage of energy from the core to the environment.
Conventionally, shielding has been in the form of a suitable metallic braiding which has been woven about the core or a suitable metal foil which has been wrapped spirally thereabout. These kinds of shielding are discussed by Henry W. Ott in his book entitled "Noise Reduction Techniques in Electronic Systems" which was published by John Wiley and Sons in 1976 and which is incorporated by reference hereinto.
Metallic braiding is relatively expensive and its use causes a diameter buildup which may be larger than desirable in particular applications. Also, it provides less than full coverage of the core and is less effective than a foil-type shield, particularly at higher frequencies. The braided shield, which has been relied on by some concerns in the field, requires the use of a relatively low line speed during the application of the braid to the core.
Generally, the metal for foil-type shields is faced with a plastic material such as MYLAR.RTM. plastic film which is used to strengthen the relatively thin metal and allow it to be processed. When such a material is wrapped about a cable core and overlapped, metal-to-metal contact at the overlapped seam is absent. This detracts from the shielding capability of such an arrangement.
The problem of insuring metal-to-metal contact of portions of the shield has been overcome in at least one prior art small pair size shielded cable. A longitudinal seam in a metallic-plastic foil laminate is formed by bending one longitudinal edge portion into a retroflexed configuration to engage the metallic portion of the one longitudinal edge portion of the laminate with the metallic surface of an opposing longitudinal edge portion. Although this arrangement provides metal-to-metal contact at the seam, it is expensive to manufacture because of the special forming of the one longitudinal edge portion.
Although the problem of metal-to-metal contact at the seam of longitudinally seamed, small pair size foil-shielded cables has been overcome, there use is not widespread. It has been believed that a longitudinally seamed cable requires the shield to be corrugated to prevent buckling. To be corrugated, the shielding material must have a thickness greater than that necessary for the shielding function. Accordingly, to avoid a somewhat bulky cable, some concerns have chosen not to use a longitudinally seamed shield.
A further problem that has arisen with the prior art braided and foil-types of shields has been the adverse effect on shielding caused by the cable having been flexed. Flexing which occurs, for example, when the cable is routed in the field causes tension and compression in different portions of the cable cross-section. Should an overlapped seam of a longitudinally seamed cable coincide with a plane of tension when the cable is flexed, the seam portions will tend to separate thereby providing a leakage path for energy from external sources and from the cable to the environment. This results in a diminution of the shielding capability of the cable.
Some use has been made of a helically wound shield for a frequency in the range of about 1 megahertz (MHz). For example, the prior art includes U.S. Pat. No. 3,274,329 which discloses a cable having a pair of spirally wrapped shields. Although flexing is not troublesome to this arrangement, a spirally wrapped shield is more difficult to terminate than one which is longitudinally wrapped about a core. Also, the spiral wrapping of the shields to stagger the overlapping portions is difficult to control from a manufacturing standpoint.
Although spirally wrapped foil is a less expensive type of shielding than braid, it has a limited effectiveness at relatively high frequencies. Frequency ranges substantially above 1 MHz are commonplace today in applications such as in private branch exchanges and in pulse clocks associated with electronic equipment. For these frequencies, leakage occurs in the commercially available foil-type shields, whether spirally or longitudinally applied. As the demand for so-called electronic wiring increases, it becomes necessary to be able to manufacture cables which provide suitable shielding over a wide frequency range.
It appears that the prior art is lacking in a cable which provides shielding against external disturbances over a relatively wide frequency range. Further, there doesn't appear to be an economical shielding arrangement which has substantial integrity notwithstanding the flexing of the cable.